Electromechanical scanning devices, such as SPMs and force scanning probe microscopes (FSPM), are commonly used to generate an image of a characteristic or sample using a probe that scans or otherwise interacts with the sample. A scanner moves the probe and/or sample relative to one another in a horizontal or X-Y plane to position the probe at a desired location on the sample, and the probe and/or sample are moved relative to one another in a vertical or Z direction to take the measurement. The scanner may position the probe at one or more discrete positions on the sample surface to take the desired measurement(s) or may move the probe back and forth across the sample in a pattern commonly known as raster scan. The probe is moved relative to the sample in some instruments, while the sample is moved relative to the probe in other instruments. In still others, the scanner includes separate actuators for translating both the probe and the sample. For example, in some instruments, an X-Y actuator translates the sample relative to the probe, while a Z actuator translates the probe relative to the sample. The scanner is typically mounted in or on a scan head or stage that supports the scanner and occasionally other devices such as components of an optical microscope. The interactions between the sample and the probe are recorded as a function of the position of the probe, and these recorded interactions are used to capture data representative of a portion of the sample.
Since the interactions of the probe and the sample are recorded as a function of the position of the probe, the position of the probe must be also be precisely known during each sampling of the sample. When imaging on the atomic and subatomic scale, the displacement between each sampling position is extremely small and, as such, highly accurate knowledge of the position of the probe is required.
Some SPMs use open loop control to position a probe at the desired location on the sample, using the voltage supplied to scanner(s) of the SPM to determine the location of the probe with respect to the sample. Others replace or supplement this open loop control with a closed loop control that utilizes scanner displacement measurements. While a number of sensors and measurement techniques have been developed to precisely measure the displacement of the probe during the scanning of a sample, these sensors suffer from a number of drawbacks. For instance, many sensors have been found to lack the necessary sensitivity for atomic and subatomic probes, are unduly large or have a limited sensing range, or are highly susceptible to interference that can negatively impact the precision of the displacement measurements.
Thus, there remains a need in the art for an apparatus that provides highly precise measurements of the displacement of the probe of an electromechanical scanning device during the scanning of a sample.